Areally and volumetrically extensive carbonate platforms are conspicuously absent from most modern eastern ocean basins where surface circulation brings cool waters from high latitudes and atmospheric circulation induces coastal upwelling. Carbonate platforms fail to develop in such settings as a consequence of sea-surface temperature instability, nutrient excess resulting in high biotic surface productivity, and oxygen-depleted conditions associated with upwelling and oceanic surface circulation. In contrast to modern patterns, extensive Paleozoic carbonate platforms developed along the eastern boundary of the paleo-Pacific Ocean (western margin of North America). Tectonic and sedimentologic evidence suggests that marginal oceanographic barriers (island arcs, thrust belts, suspect terranes) existed during much of this interval. These barriers effectively protected landward carbonate platforms from the adverse conditions typical of open eastern ocean basins. Without these barriers such extensive platforms may not have developed. In regions that lack evidence for structurally or tectonically controlled highlands, the existence of eastern ocean basin carbonate platforms may serve as an important clue in paleogeographic reconstructions of the continental margin.